1. Field of the Invention
The present invention relates to an image processing apparatus for processing an image signal.
2. Related Background Art
Dither method and density pattern method have been well known as techniques to represent a half-tone image. In either case, however, sufficient tonality is not attained by a threshold matrix of a small size. A larger size threshold matrix may be used but, in this case, resolution is significantly lowered.
On the other hand, a technique to improve tonality by a relatively simple apparatus while maintaining a high resolution has been proposed by the assignee of the present invention. In that technique, when a digital image signal is binarized and an image is formed by a laser beam printer, the digital image signal is converted to an analog signal which is compared with a periodic pattern signal such as a ramp (triangular) signal to produce a pulse-width modulated binary signal, which is used as a drive signal for a laser light source so that the tonality for the half-tone image is improved. The analog image signal, ramp wave and the binary signal resulting from the pulse-width modulation are shown in FIG. 10.
By pulse-width modulating the digital image signal, the high resolution and the high tonality are compatible. In such a conversion, if a tonality which exactly represents that of an original image is to be reproduced, compensation must be done while taking an input characteristic of a document reader and an output density characteristic of a printer into consideration.
FIG. 11 shows a block diagram of an image forming apparatus having a .gamma. correction (tonality correction) function, by a pulse width modulation system proposed by the assignee of the present invention.
Optical image information reflected by a document (not shown) is converted to an analog electrical signal by a CCD 111. The analog electrical signal from the CCD 111 is amplified to an appropriate level by an amplifier 12, and the resulting analog signal is converted to a digital signal by an A/D converter 113. A tonality (.gamma.) correction circuit 114 corrects a variation of tonality of the digital signal between an image input and an image output. Usually, the input tonality signal is converted to a corrected tonality signal by referencing a look-up table which contains tonality correction constants of a system stored in a ROM. The corrected digital image signal is again converted to an analog signal by a D/A converter 115, and the resulting analog signal is compared with a ramp wave signal generated by a ramp wave generator 117. Numeral 116 denotes a comparator, an output of which is a binary image signal which is pulse-width modulated with a density. The binary image signal is supplied to a printer 118 to control turn-on and turn-off of a laser light emission so that a half-tone image is produced. Namely, the half-tone image is formed by controlling light emission period of the laser.
Major factors to determine the .gamma. characteristic are an input characteristic (a characteristic for converting the output of the CCD 111 to the analog electrical signal) and an output characteristic (a density characteristic to form a final record image from the pulse width modulation of the printer 118). The CCD 111 usually has a monotonous characteristic relative to light intensity, and the printer 118 has one of various characteristics depending on the system.
FIG. 12 shows a typical example of the input characteristic when a laser beam printer of an electro-photographic system is used, and an example of the .gamma. correction table.
For a density in a quadrant II, the density of the CCD 111 exhibits a characteristic shown in a quadrant I. The printer characteristic is shown in a quadrant III. In order to attain an output characteristic which exactly represents an input characteristic, the density input by the CCD 111 must be .gamma. -corrected as shown in the quadrant III. Thus, a correction table shown in a quadrant IV is provided in the tonality (.gamma.) correction circuit 114 of FIG. 11.
As seen from FIG. 12, in order to attain a linear tonality shown in the quadrant II, it has been found that there is a factor which causes a significant quantization error in the digital input-output characteristic shown in the quadrant IV and a remarkable pseudo-outline is created in a low density area. Namely, a gradient of the digital input-output characteristic in the low density area in the quadrant IV is very low, that is, approximately 1/5 to 1/10 of a theoretical gradient. Accordingly, even if there are 64 tonalities in the input image, the output image has only 12 tonalities, or in a worst case, 6 tonalities. If the gradient is 1/5 of the theoretical one, it means that the output changes by one step when the input changes by five steps. As a result, the reproducibility of the tonality is reduced by a factor of five by the quantization error. It cannot be said that the CCD's or printers of the same type have no variation, and when such variations are accumulated, different output images are formed for one document.
As described above, the techniques to reproduce a high quality image by using ramp wave or table are disclosed in U.S. Application Ser. No. 029,386, U.S. Pat. No. 4,679,074, U.S. Application Ser. No. 051,154, U.S. Application Ser. No. 770,770, U.S. Application Ser. No. 870,421, U.S. Application Ser. No. 900,603, U.S. Application Ser. No. 919,763, U.S. Application Ser. No. 010,539, U.S. Application Ser. No. 923,026, U.S. Application Ser. No. 932,030, U.S. Application Ser. No. 012,322 and U.S. Application Ser. No. 013,629.
However, an image processing apparatus having a higher quality of reproduced image has been demanded.